A radiation therapy device typically includes a radiation delivery device mounted to a gantry that is swiveled around a horizontal axis of rotation in the course of a radiation therapy treatment. The radiation delivery device generally delivers a high energy radiation beam. During treatment, the radiation beam is directed towards a patient lying in the isocenter of the gantry rotation.
The device thus normally includes a radiation source, such as a linear accelerator, for supplying the high energy radiation beam. The high energy radiation beam is typically an electron beam or X-ray beam.
To control the radiation emitted toward a given object, a beam shielding device, such as a plate arrangement or a collimator, is typically provided in the trajectory of the radiation beam between the radiation source and the patient. A collimator is a computer-controlled mechanical beam shielding device which generally includes multiple leaves, for example, a plurality of relatively thin plates or rods, typically arranged as opposing leaf pairs. The plates are formed from a relatively dense and radiation impervious material and are generally independently positionable to size and shape of the radiation beam. These leaves move over the tissue being radiated, thus blocking out some areas and filtering others to vary the beam intensity and precisely distribute the radiation dosage.
A multileaf collimator (MLC) is an example of a multileaf beam shielding device that can accurately and efficiently adjust the size and shape of the radiation beam. The size and shape of a radiation beam is designed during the treatment planning process. This is useful for both intensity modulated radiation treatment (IMRT) and three-dimensional conformal radiation therapy (3D CRT).
Traditional radiotherapy utilizes uniform beams of radiation, producing a uniform distribution of dose throughout the irradiated volume, which includes the target volume. This ensures the target is adequately covered, but does little or nothing to avoid often critical surrounding structures. With IMRT, the beams of radiation are made to be intentionally non-uniform. In this manner, the dose distribution can be shaped to reduce or eliminate radiation to surrounding structures. As a result, IMRT is increasingly used to treat large volumes because IMRT can deliver more conformal radiation while sparing the surrounding normal tissue.
Monitor unit (MU) efficiency is a commonly used measure of beam efficiency. MU efficiency is defined as the efficiency with which the incident radiation results in dose being in absorbed in the target region of a patient. A consequence of low MU efficiency is an increase in leakage radiation that reaches the surrounding (normal) tissue of the patient.
There are several components of a successful IMRT program. The first is a process referred to as “inverse planning.” Inverse planning utilizes a mathematical algorithm to optimize the intensity of the various beams. This optimization process typically is highly computer intensive.
The second component is a process to convert the intensity distributions obtained, often referred to cumulatively as a fluence map, into a series of MLC leaf movements. This is referred to as “leaf sequencing.” Many device-specific factors must be accounted for in this process. These factors include radiation leakage through and between the leaves, leaf speed, dose rate, and the “tongue-and-groove” effect.
IMRT can be performed either while the beam is on, which is referred to as dynamic multileaf collimator (DMLC) delivery, or by turning the beam off while the leaves move to their next position, which is referred to as segmented multileaf collimator (SMLC) delivery. The beam shielding device defines a field on the object to which a prescribed amount of radiation is to be delivered. The usual treatment field shape results in a three-dimensional treatment volume which includes segments of normal tissue, thereby limiting the dose that can be given to the target, such as a tumor. The dose delivered to the tumor can be increased, thereby decreasing the treatment time so that the amount of dose delivered to the normal surrounding tissue is decreased. Although current leaf sequencing algorithms have reduced somewhat the radiation level reaching surrounding normal tissue as compared to traditional uniform beams of radiation, these leaf sequences have not provided optimal MU efficiency.
Most IMRT treatments are administered with conventional MLC systems that are typically available on commercial linear accelerators. The MLC systems vary in design but each version has certain mechanical limitations, such as maximum leaf over-travel which limits the attainable width of the radiation beam.
It is sometimes necessary to expose large areas of the body of a patient to radiation. If the size of the required radiation field is too large relative to the maximum attainable width provided by the radiation delivery system, such as in the case of a large tumor, the entire radiation field cannot be exposed at one time by the radiation system. This necessitates that a large field be split into a plurality of abutting field portions, such as 2 or 3 fields portions, where the respective field portions are delivered one at a time.
The methods currently used for field splitting generally split the overall field into field portions having equal width. Thus, the width limitation problem is solved without regard to efficiency, and generally results in relatively poor monitor unit efficiency. This often results in longer delivery times, poor MU efficiency, and field matching problems.
Specifically, uncertainties in leaf and carriage positions have been reported to cause errors in the delivered dose (hot or cold spots) along the match line of the abutting field portions. Differences of up to 10% along the field split line when the split line crossed through the center of the target for all the fields has been observed.
Proposed solutions to the problem of dosimetric perturbation along the field split line include automatic feathering of split-fields by modifying the split line position for each gantry position or by dynamically changing radiation intensity in the overlap region of the split fields. However, none of the field splitting techniques reported have disclosed treatment delivery and MU efficiency optimization for split fields.